1. Field of the Invention
The present invention relates to electrical cables, with an improved semiconductive insulation shield and the method of making the same. More specifically, the invention is concerned with an electrical cable with a foamed semiconductive insulation shield which serves as both a cushioning layer and an electrical shield. Preferably, the foamed semiconductive insulation shield is a closed-cell foamed semiconductive insulation shield.
2. The Related Art
Electric power cables for medium and high voltage typically include a central core electrical conductor of copper or aluminum, an overlaying semiconductive conductor shield, an electrical insulation layer formed over the conductor shield, a semiconductive insulation shield and a metallic shield overlaying the insulation shield. Preferably, an overall plastic jacket is positioned radially external to said metallic shield. The thickness of each of these layers is determined by voltage rating and conductor size and is specified by industry standards such as those published by the Insulated Conductors Engineering Association (ICEA), the Association of Edison Illuminating Companies (AEIC), and Underwriters Laboratories (UL). Electrical cable performance criteria are specified and tested according to AEIC and ICEA standards. The conductor shield is most often a semiconducting polymer extruded over the electrical conductor. The insulation layer is usually a thermoplastic or thermoset material such as crosslinked polyethylene (XLPE), ethylene-propylene rubber (EPR), or polyvinyl chloride (PVC). The insulation layer may include additives to enhance the life of the insulation. For example, tree retardant additives are often added to XLPE to inhibit the growth of water trees in the insulation. The insulation shield is usually an extrudable semiconducting polymer. The insulation shield must have a smooth interface with the insulation layer and exhibit an acceptably low voltage drop through its thickness and eliminate discharge. The AEIC specifies that the insulation shield must have a volume resistivity of less than 500 Ω·m (Ohms×meters) at 90° C. and 110° C. Insulation shields usually form a layer which is adhered to the insulation layer, or for high voltage cables, bonded to the insulation layer. The metallic shield overlaying the insulation shield may consist of, for example, a lead or aluminum sheath, a longitudinally applied corrugated copper tape with an overlapped seam or welded seam, helically applied wires (i.e. drain wires or concentric neutral wires), or flat copper straps. It is important that the insulation shield be in electrical contact with the metallic shield. U.S. Pat. No. 5,281,757 (hereinafter the '757 patent) and U.S. Pat. No. 5,246,783, the contents of both of which are incorporated herein by reference, disclose examples of electric power cables and methods of making the same.
There is sometimes a semiconducting tape layer interposed between the insulation shield and the metallic shield. The purpose of this tape may be for waterblocking, cushioning, or both. If for cushioning or bedding, this tape most often is employed in conjunction with a metallic shield comprised of lead or aluminum sheaths, copper tape with or without welded seam or with sealed overlap longitudinally applied corrugated copper tapes as in the heretofore referenced '757 patent. The cushioning effect of the tape layer eases the pressure on the metallic shield due to the expansion and contraction of the electrical cable core resulting from varying load cycles on the cable. The use of cushioning or bedding layers are known under concentric neutral wire metallic shields; however, due to the expansion and contraction of the electrical cable core, the concentric neutral wires often indent the insulation shield. This indent is sometimes transferred to the insulation layer, causing a disruption of the cylindrical interface between the insulation shield and the insulation layer. This disruption leads to higher electrical stresses as well as to detachments of the semiconductive insulation shield from the electric insulation layer, which may result in premature failure of the insulation layer and the cable. The cushioning layers hereinbefore described add an expensive component to the cable and add an additional manufacturing step.
When splicing or terminating prior art electrical cables, the metallic shield is removed from the splice/termination area. Conventional splice and termination sleeves have portions that compress around the insulation shield. When the splice or termination is completed, a large void is present between the sleeve and the insulation shield because the insulation shield does not compress. These voids, if not properly or completely filled with a grease, can cause failure of the splice or termination due to partial discharge which will eventually erode the insulation layer.
U.S. Pat. No. 4,145,567 to Bahder discloses an electric power cable which employs a semiconducting compressible layer of closed-cell foamed plastic extruded over the insulation shield and under a metallic shield comprised of a longitudinally folded tape with bonded or welded overlap seam. As the cable core becomes highly heated, it expands and increases in cross-section. The compressible layer between the insulation shield and the inside surface of the metal shield accommodates the expansion of the core by decreasing in radial thickness. When the cable core cools, the compressible layer expands again, so that it maintains contact with the cable core and the metal shield at all times. In this way, the pressure exerted by the compressible layer against the insulation shield and the metallic shield is sufficient to prevent any flow of fluid lengthwise of the cable if the metal shield becomes punctured by lightening or other cause. Examples given for Bahder's compressible layer are EPR which is either semiconducting when used with a copper metallic shield or filled with high dielectric constant fillers such as titanium dioxide, barium titanate, or magnesium zirconate. Furthermore, according to Bahder extruding the compressible layer is an additional manufacturing step and the problem of voids in splices and terminations is not alleviated. Moreover, the compressible layer disclosed by Bahder functions as a cushioning layer which is used in electric cable constructions in addition to an insulation shield.
Document WO 99/33070 in the name of the Applicant describes the use of a layer of expanded polymeric material arranged in direct contact with the semiconductive insulation shield of a cable, in a position directly beneath the metallic screen of the cable, and possessing predefined semiconductive and waterblocking properties with the aim of guaranteeing the necessary electrical continuity between the conductor and the metallic screen.